It is conventionally believed that vessels (ships) with double steel hulls or double bottoms have a better chance of avoiding cargo spills during accidental groundings or collisions, as compared with vessels constructed with a single steel hull. The conventional wisdom is that, although the outer hull may be penetrated during an accident, the inner hull is likely to remain intact and, therefore, the liquid cargo, being enclosed by the inner hull, will not leak to the sea. The marine industry, however, remains fairly evenly divided on the subject. Some arguments challenging the effectiveness of double hulls and/or double bottoms, other than their cost, include the following.
Double hulls can be effective only during "low energy" groundings or collisions. A void space of only ten to twelve feet between the inner and outer hulls will be ineffective to prevent penetration of the inner hull during more severe grounding accidents or collisions. During a grounding, flooding of the void space in the double bottom or double hull with sea water will cause the ship to sink deeper onto the rocks or may even cause the ship to break in two, spilling most or all of the cargo. Salvage attempts on a ship with flooded double bottoms will be seriously hampered because of this sinking effect. Double-hull ships present additional safety problems due to the increased possibility of accumulation of explosive vapors within the enclosed void areas between the inner and outer hulls. Maintenance and inspection of the narrow spaces between the two hulls will be difficult and since these spaces normally will be used to carry water ballast, there are increased risks of undetected corrosion damage. Due to added steel weight, double hulls result in a loss of cargo carrying capacity as compared with a conventional single hull ship of equal volume. Apart from loss of income to the shipowner, this loss of capacity also results in more ships in ports and sea lanes to carry the same amount of oil and, consequently, increased chances for accidents and groundings.
Notwithstanding the above arguments, the "U.S. Oil Pollution Act of 1990" recently was signed into law, mandating that all new tankers operating within the 200-mile U.S. Exclusive Economic Zone (EEZ), be of double hull construction. The Act includes a phase-out schedule for all existing single hull tankers between the years 1995 and 2010. Some countries outside the U.S. appear ready to follow with similar laws. It is estimated that only 3% of the world's tanker fleet currently complies with the requirements of the "U.S. Oil Pollution Act of 1990". Since retrofitting existing tankers with double steel hulls is considered technically and economically impractical, it follows that only newly-built double-hull tankers may serve countries with such laws in the future. If one accepts the argument that "double hulls" spell " double trouble", one is faced with the requirement virtually to rebuild the world's tanker fleet with ships that may prove prone to more and larger oil spills.
It will be noted that in view of the new legislation prompted by recent accidents and the debate over double hull effectiveness, many innovative techniques have been suggested by the marine industry to prevent or limit oil spills when a hull breaching accident occurs. Most of these suggestions deal with hydrostatic equilibrium of cargo and sea, or applying vacuum to cargo tanks or similar combinations. It is generally admitted, at least during the first few minutes of breaching, that some oil may escape to the sea with any of these designs.
The present invention provides an improved arrangement for preventing oil spills. Additionally, apparatus constructed according to this invention may be easily and cheaply installed (retrofitted) in existing single hull tankers, making them effectively double-hulled. A flexible inner hull resists breaching far better than a rigid steel hull. At the same time, all the aforementioned disadvantages of steel double hulls are substantially avoided.